1. Field of the Invention
This invention relates to a mirror comprising a transparent glass sheet bearing a copper reflective layer and at least one protective layer, and to a method of manufacturing a mirror comprising forming on a transparent glass sheet a copper reflective layer and at least one protective layer.
2. Description of the Related Art
Mirrors bearing a silver reflective coating are of course extremely well known. Such mirrors in general have a neutral tint in reflection, that is, they do not appreciably modify the colour of light which they reflect. It is occasionally desired to make use of a mirror which does modify the colour of the light it reflects. This effect could be achieved by using coloured glass for forming a silver mirror, but a more usual course is to make use of a reflective layer which colours the reflected light. A gold reflective layer deposited on a clear transparent support imparts a tone to reflected light which is thought by many to be aesthetically pleasing.
Known methods of depositing a reflective layer of gold on a transparent support tend to be rather expensive, and gold is itself a costly material. Various proposals have therefore been made to mimic the appearance of a gold mirror by manufacturing a copper mirror, that is, a mirror with a reflective layer of copper. Copper as a material is less costly than gold, and it can readily be deposited by chemical methods as opposed to the more expensive vacuum deposition or sputtering techniques.
Unfortunately, a copper reflective film has the disadvantage of being rather easily corroded by the atmosphere, and particularly by pollutants such as sulphur dioxide, despite the presence of a protective layer for example of paint. As a result, the thin reflective layer of a copper mirror discolours (blackens) in the course of a fairly short time. This is of course quite unacceptable to the user.
This problem has received much attention in the past, and various proposals have been made to protect a reflective copper film against discoloration by applying various protective materials in efforts to prevent or retard atmospheric corrosion. To date, the most effective of these proposals has been to treat the copper layer with substituted azole. Such proposals are for example set out in British Patent Specifications Nos. 1,074,076 (Pittsburgh Plate Glass Co.), 1,250,142 (Shikoku Kasei Kogyo Co. Ltd.) and 2,102,453 (Glaverbel), and in U.S. Pat. No. 4,255,214 (Falconer Plate Glass Corp.). A said protective layer may thus comprise a superficial stratum of the copper layer which has been contacted by azole. While such azole protection is undoubtedly beneficial in retarding corrosion of copper films and enables formation of the best chemically deposited copper mirrors hitherto available, it has been found that even azole protected copper films may change in aspect and also that during fabrication of the mirror, even in a short time before a substituted azole is applied to the mirror, changes in aspect are observed. It is for this reason that copper mirrors have hitherto been difficult to market on a commercial scale.
An indication of the resistance to aging of a mirror incorporating a metallic film can be given by subjecting it to a copper-accelerated acetic acid salt spray test known as the CASS Test in which the mirror is placed in a testing chamber at 50.degree. C. and is subjected to the action of a fog formed by spraying an aqueous solution containing 50 g/L sodium chloride, 0.2 g/L anhydrous cuprous chloride with sufficient glacial acetic acid to bring the pH of the sprayed solution to between 3.0 and 3.1. Full details of this test are set out in International Standard ISO 3770-1976. Mirrors may be subjected to the action of the saline fog for different lengths of time, whereafter the reflective properties of the artificially aged mirror may be compared with the reflective properties of the freshly formed mirror. We find that an exposure time of 120 hours or longer gives a useful indication of the resistance of a copper mirror to aging.